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Oil temp gauge / oil thermostat problems

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Perhaps Mr. Mortensen can chime in on this - or anybody for that matter.


I purchased an oil cooler thermostat and a sending unit housing from Pegasus. I've had the car up to operating temperature numerous times. The oil temp gauge will not budge, nor am I seeing the oil thermostat opening.


Is it possible for the sending unit "sender" to go bad? Any way to test it? The gauges were already in the car when I bought it.


I can touch the sending unit housing and the thermostat themselves and they are indeed warm/hot to the touch. Oil is definitely flowing through those pieces. The hoses after the thermostat, however, are stone cold. Telling me the thermostat is not opening. I made all connections per the install instructions provided. I suppose I could have it backwards, but I triple checked.


The routing goes filter adapter out > Oberg oil filter > temp sensor housing > thermostat > oil cooler > thermostat bottom side > oil filter adapter in.


I find it strange that these two problems coincide, yet I suppose anything is possible.

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Okay - it seems awful strange that nobody has ANY input on this. Am I a dorkhead asking newbie questions that I should be able to search on? (More than likely!)


I found out that by crimping or damaging the sending unit near the sensor bulb - or creating a harsh bend can cause the sending unit to fail. The gauges are activated by a gas inside a copper tube that expands as it heats and drives the gauge. Since the gauge was already in the car, I can surmise that the sending unit is bad and the PO ruined it when tearing out his old engine. I'm ordering a new gauge.


The kicker is - I have pulled the outlet hose from the oil thermostat, and oil is indeed going through the thermostat (it pooled up real nice in my garage), and I also made sure that the bolts are facing the bottom per MoCal's instructions. I am still unable to get any oil to the cooler. Is the thermostat directional? Meaning, I could have installed it with the bolts facing downward, but just installed it backwards?


JohnC recommended this device - wondering if he has any input? I've driven the horse p** out of the car and gotten it well into operating temperature. Not sure what's going on. Guess I won't be able to tell until I get a new gauge huh?


Thanks all....

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  • 2 weeks later...

OK - kudos to me for figuring this out....BUT have a question. Turns out the gauge WAS working. I put in a new one and still got no reading on oil temp after 20 min. of driving. I pulled the sensor bulb and hit it lightly with a torch, and sure as heck - the gauge reads.


My question is - how hard do I have to drive the car to get the oil up to 180 degrees so the thermostat will open? I have done some pretty "spirited" sessions and can never get the gauge to budge - and hence the thermostat never opens because it's set to open at 180 degrees. Is it normal for my oil temp to be this low after driving fairly hard?

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I am running a Perm-A-Cool oil thermostat with a front mounted Oil Cooler and Autometer oil temperature gauge which has the probe threaded into the oil pan.


To answer your questions, usually within 5 minutes my gauge starts coming off the peg and usually within 15-20minutes its at 180ish F on the gauge.


My set-up:




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Yasin - first off - that is a NICE setup! Secondly, I have my probe mounted in a special AN threaded holder that is right up on my right inner fender near the core support. It's mounted after the remote oil filter. The oil then goes about 2" further into the thermostat. I did not mount mine in the pan. I got the remote mount from Pegasus Auto Racing. I still feel the lines are hot, but apparently not 180 degrees worth of hot.


I guess I won't worry about it too much. I'm not running a turbo like you, so maybe my oil just plain isn't going to get hot enough to cause a problem - much less open the thermostat.

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Thanks! If your probe is monitoring moving oil, it will be hard for it to register a reading (I am pretty sure this is your problem), whch is why the pan is the best location. To me it was worth the effort to drop the pan run it over to my fabricators and let him weld a bung for the oil temp sensor.


If you had a digital pyrometer, you could always verify the temperature of the lines and that would give you an indication of oil temp.


Regards - Yasin

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I've got the temp sensor mounted just before the thermostat. If the "moving" oil is not going to get enough temp. to move the gauge, it's certainly not going to get enough temp to trip the thermostat - right? At that point, why even bother having a thermostat. John Coffey recommended (highly) that I get the thermostat. If my oil is never going to get hot enough to trip the gauge, why even bother with a thermostat? It'll NEVER trip it. I wonder why he recommended that if the moving oil is a "known factor"?


It's not like you mount the thermostat to the oil pan. John specifically recommended Oil filter adapter > remote oil filter > thermostat > cooler. I figured if I put the temp sensor just before the thermostat, I'd know exactly what is going on at all times. I DO have the AZC oil pan with the turbo drain tube plugged off. Perhaps I pull that and install the sensor there? Although, it's high enough on the pan that perhaps it never touches oil - hence the "drain" tube inlet....Oh - isn't this soooo much fun?


Yasin - I really appreciate your help.

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What you have is a low resolution temperature sensor. It will only give you a general idea of what is going on with the oil temperature. Low mounting in the sump may give you an idea because it's bathed in the oil more directly. Trying to get a 'flowing' measurement is difficult without disrupting flow through the line.


Normally people will monitor sump temperature by a bulb in the sump. And where you are monitoring it, technically that is what you are seeing I suppose.


Thing is, your thermostat is a 'bypass through the cooler' setup, to it maintains a constant temperature downstream of the thermostatic valve.


In theory you should see a rise in the sump temperature as the constant temp out of the thermostat would be subject to different load issues in the engine and drain back to the sump at different temps at different times.


To get the oil at a constant temperature, the thermostat will not direct a constant flow through the cooler, but rather let the oil in it sit for as long as possible and cool down as much as possible, and 'remix' your hot oil from the sump with super cool oil from the cooler return to get a given temperature downstream to the engine. If you have a highly stressed engine then the flow through the cooler would be more. During low stress engine operation likely you will have two hot lines and two cool lines. The hot lines will be 'in and out' of the thermostat in the direction of the oils flow to the bearings as the cooler may not be in play at all and the lines to and from it may have time to cool completely.

During extreme stress (high speed driving up a long grade let's say)---then you will likely have THREE hot lines: To the thermostat, to the cooler, and to the engine, with ONE cold line---that FROM the cooler back to the thermostatic body for the Mixing feature.


In most cases you will be able to feel this with your hands, but some times you will need an I-R Thermometer to differentiate the temperatures that are happening.


The worst case is that all the hoses are identical in temperature, and the oil is something like 240+F. Then that would tell you your engine is highly stressed and producing heat that your cooler can't sufficiently reject.


The immediate precursor to this situation is that you will start seeing high oil temps into the thermo body from the engine, to the cooler, and from the cooler it's still relatively cool, but even with the thermo mixing valve all the way open letting that cool oil mix with the hot from the engine...it still isn't making it back to the design thermostatic setpoint.


Normally you would see 180-220 oil temperature. Examples of each situation follow:

220 from sump, out of pump, to Thermo Body. To Cooler hose may be anywhere from 220 on down to close to 100F. From Cooler hose will be something like 10 degrees above ambient temperature (say 80F on a 70 degree day). From Thermostat hose to the engine will be in the area of 180.


On a stressed run, if you are datalogging, you might see 220 in sump, 220 to cooler, 80 from cooler, and 180 to engine.


Overloading the engine and oil cooler's capacity (again if you are datalogging), it may be 235 and rising in sump, 235 to cooler, 190 to the thermostat and 200 to the Engine.


Worst case is where you have totally overloaded the whole schebang, and you see 275+ in the sump, 275 to the cooler, 200 from the cooler to the thermostat, and 245 to the engine.


There should usually always be a differential across the cooler as long as you are running sufficient oil flow through it, and cooling medium across it. Really the differentials across each part of the system would be the same if it was running at constant load and everything ws in equlibrium, but unless you are in a stationary powerplant you likely won't see it.


Unless you have K-Style Thermocouples strapped to the various points to do some checking while you're actually working the engine it may become very difficult to diagnose someting wrong. If you have a LARGE oil cooler, the flow through it may indeed be VERY LOW as you will cool that oil to near ambient, and if you only have say 200 degree oil in the sump it will not take a whole lot of flow mixing 70 degree oil to drop that 200F to 180F in the line to the engine. (Check out the thread on EGT monitoring for some links to K-Thermocouples and multiple readers that can be used for this kind of check.)


This can also happen at idle.


In both of these cases, you will have 'two hot and two cold' lines. it will LOOK as if the thermostat isn't doing anything, but in reality it only really works when you are really loading up the engine.


If you are driving in 60 and low 70 degree weather, you may not see 180F in your oil. A large sump can make it take a while to come up to temp as well. If you are running a 160F thermostat in your water system, it may take a while for the oil to get beyond 180 as well. Oil temperatures usually end up 20-40F higher than your water temperature, and react quicker to loads than the water system. When you have 275F oil temperature, you may well expect to have some water cooling issues unless you have a good cooling system and a high pressure radiator cap!


I hope that made some sense to explain how it all works.

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Tony - as always - succinct and to the point. Yes - I understand completely. I haven't really taken the car out for more than 30 min. on any given stretch, and the weather has been fairly cool. That would explain why I'm not getting the oil up to temp. I got one of these;




to mount the sensor bulb in-line. It is mounted after my Oberg filter and before the thermostat. Now that you mention it, on the last run I did, I was feeling a little heat on the "send" side from the thermostat TO the oil cooler, but the return line was cold. Your explanation made sense of that. Guess I just need to drive harder eh?


So - is the inline gauge adapter not going to work for me? Is it best to mount the sensor in the pan? I can't imagine they'd sell a whole heck of a lot of these things if they weren't meant for a purpose.


thanks again!

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In either location---sump or the pump outlet to the thermostat, as long as the sensing element is bathed in flowing oil, it should read almost the same temperature. Sometimes the temperature out of the pump is slightly higher at the oil filter due to more heat picked up in the block, but we are talking 3-5 degrees in this case. Nothing the gauge will show, but that a digital thermocouple readout would.


That location should be fine.


Drive harder. It will take 30 minutes + in cool weather to get up to oil temperature.


A strange thought on this, I just bought an oil pan heater, as well as a block heater for the Dually. Not that it gets that cold here....but starting that baby stone cold (30 to 40 degrees) means a LOT of water needs to be heated up. If I heat the oil and the water, I spend less time (if any at all) on the cold-start circuit.


With a 7.4 Liter Dually....used mostly for short trips....that can mean a significant increase in mileage. My testing has shown thus far that I go from a solid 8mpg to 10mpg (almost 11) by preheating the water alone. Which is what I get when tooling along warmed up at 65mph unloaded.


The oil heater is just bitchen cool to have, we used them on diesels I used to work on, and they make nice gear heaters for assembly work since they are internally self-regulated to not exceed specific temperatures. So if the water heater gives me warmish oil from the get go, I'm keeping the oil heater for some other stuff! LOL

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Here's a shot of my setup. The sender should indeed be bathed in flowing oil. You can see the Oberg filter on the left, then the sending unit housing, then directly to the thermostat. Second pic is the hose coming out of the thermostat to the cooler up front. So, this should indeed work - correct?






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Yeah - Tony nailed the problem.....Drive Harder!!! He stated as long as the sensor was bathed in hot oil, it should not matter where it's located. It is indeed bathed in hot moving oil, so I just need to drive it harder and for a longer duration to ensure that it gets up to temp. I haven't been to the track yet, but I suspect it will start registering then - if not sooner when I get the chance to drive it for an extended period of time. The hoses definitely heat up, and when I pull the bulb sensor, it is indeed warm and dripping with oil. Just not up to 140 degrees I would gander...

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